The present invention relates to a new electrical resistance-heating element of the molybdenum silicide type intended for use in connection with sintering of metal powder.
Resistance-heating elements of the above-mentioned kind have existed since the 1950""s such as the element sold under the trademark Kanthal Super. These elements are usually made of a metal-like phase of MoSi2, alternatively of MoxW1xe2x88x92xSi2, in addition to an oxide phase of the aluminum silicate type. Similar types of material can be used in an oxidizing atmosphere at element temperatures up to 1900xc2x0 C.
What makes this high working temperature possible is, besides the high melting point of the material (over 2000xc2x0 C.), is the formation of an oxide layer of SiO2 on the material which rapidly makes the basic material passive against accelerated oxidation and thus a long service time for the heating element is possible. This outer layer gives lasting protection in several furnace and heat treating atmospheres, such as air, oxygen, nitrogen/ hydrogen gas, cracked ammonia, and others. Conditions that often limit the use of such materials include high temperatures of the element in conjunction with an inadequate potential for oxygen, or a dew point that is too low. If the critical proportions between the dew point and the temperature of the element is exceeded the SiO2-layer becomes unstable and, after a certain time, does not give any protection to the base material. For instance in hydrogen gas this occurs at an element temperature of 1300xc2x0 C. when the dew point is lower than about xe2x88x9230xc2x0 C. To keep the SiO2 layer stable when temperature of the element is 1450xc2x0 C., a dew point over +20xc2x0 C. is required, i.e., an atmosphere containing more than 2.3 percent by volume of water. The instability of the SiO2-layer poses a restriction of the usage of the element in certain connections.
An application where such limitations manifest themselves is the sintering of metal powder in order to produce stainless steel. Components of the stainless steel grade AISI316L are produced by pressing of powder, or by injection moulding metal powder. After evaporation of the binding agent at a low temperature, a final sintering in the range of temperature between 1300-1360xc2x0 C. in reduced atmosphere is often required. The reducing gas can be pure hydrogen gas with a dew point at xe2x88x9240xc2x0 C. to xe2x88x9260xc2x0 C., corresponding to about 0.01 and 0.001 percent by volume of water, respectively. The low dew point has to be maintained in order to reduce metal oxides during the sintering process, which results in a material with high density and good mechanical properties. For such an application, a heating element temperature between 1400 and 1550xc2x0 C. should be required, dependent on the element shape and furnace design. Under such conditions, the SiO2-layer formed on conventional heating elements based on MoSi2 is not stable.
Heating elements which are used today in many furnaces for sintering of metal powder in the temperature range over 1250-1300xc2x0 C. are mainly manufactured from molybdenum, but some are also made from tungsten. A limitation of this material is, besides its relatively high total cost, is the requirement that furnace elements made from such materials must be kept over 400xc2x0 C. in an atmosphere deficient in oxygen in order to avoid the detrimental oxidation of the pure molybdenum or tungsten metal. Thus, furnace leakage or other breakdowns can damage such elements.
Alternative materials which exist for electrical resistance heating under these conditions are alloys and intermetallic compounds such as FeCrAl, NiCr and MoSi2 (e.g. Kanthal Super as above). The limitations of MoSi2-material were described above. FeCrAl and NiCr form oxides of Al2O3 and Cr2O3, respectively, on the surface of furnace elements made from such materials when under air. In a reducing atmosphere, such as dry hydrogen gas, the range of temperature under use is limited to about 1400xc2x0 C. for FeCrAl and 1250xc2x0 C. for NiCr (e.g.-sold under the trademark Nicrothal 80).
In case of NiCr-alloys the Cr2O3 is not stable above this temperature. In case of FeCrAl the Al2O3-layer certainly remains stable, but the service time of the material at this temperature is limited by the melting temperature which is close to about 1500xc2x0 C. Thus, if the FeCrAl should be used for sintering of 316L type material, the requirements for high element temperatures would lead to very limited service times.
It would be desirable to use a material to form such elements that combines the ability to form alumina on the surface with a melting temperature considerable higher than 1500xc2x0 C., and could be used in either a reducing or an oxidizing. Thus, the disadvantages of the molybdenum elements could be eliminated since the elements do not always have to be used in an atmosphere deficient in oxygen.
The present invention overcomes the deficiencies of the above-mentioned conventional materials, and others.
In one aspect, the present invention provides an electrical resistance heating element for sintering metal powder, the element made from a material having a composition comprising mainly a silicide phase according to the formula Mo(Si1xe2x88x92xAlx)2, which phase forms alumina on the surface of the element.
In a further aspect, the present invention provides a method according to sintering a metal powder comprising: heating the metal powder with a heating element made from a material having a composition comprising Mo(Si1xe2x88x92xAlx)2, wherein x is 0.1 to 0.6.